In general, an annular bead core, which is disposed in each bead portion of a pneumatic tire to provide a hoop effect, is made of windings of a metal wire wound at a predetermined number of time, and it has a function to secure the carcass ply by turning up the carcass ply around the bead core from the axially inside to the outside of the tire.
On the other hand, the inner surface of a tire is provided with an inner liner of air-impermeable rubber extending over the tread portion, sidewall portions and bead portions in order to maintain an air pressure. Conventionally, the radially inner end of the inner liner is terminated near the bead cores, and usually, in a region axially inside the bead core the axially inside of the bead portion is not completely covered with the inner liner. This is because of the idea that, as the bead portion is thick and its area is small for the tire as a whole, the amount of leaked air is very small and there is no substantial influence upon keeping of the air pressure.
In a heavy duty pneumatic tire for trucks and buses which is used under severe conditions at high tire pressure and heavy loads, especially which has steel cords as the carcass cords and/or a bead reinforcing layer, even if there is no substantial problem of air leakage, if small amount of air or moisture penetrates into the bead portion and reaches to the surface of such a cord on which a small separation or adhesion failure occurs, the cord will be eroded. If the cords are steel cords, the rust may be caused. If the cord is made of organic fibers, the strength may be decreased as they absorb water, and as the result, there is occasionally seen a cord damage and a separation of the cords from the topping rubber. Usually, this does not expand into the deterioration of the bead durability in the case of the light-load tires. However, in the case of heavy-duty tires, as the deformation of the bead portions and the shearing force between cords and rubber are large, sometimes the durability of bead portions is impaired.
It is therefore, an object of the present invention to provide a pneumatic tire in which the damages of the bead portions caused by the cord damage and the separation between cords and rubber is prevented, and the durability of the bead portions is improved.